The contamination of soil, aquifers and vegetation by ionic materials such as radionuclides and/or heavy metals has been a problem for decades. The danger, of course, is that humans and animals can be poisoned either directly or through the food chain. Our earlier-filed PCT Application U.S. Ser. No. 90/03997, published on 7 Feb. 1991 as WO 91/01392 ("the '1392 application"), presented several new processes and apparatuses for decontaminating a bulk source such as soil or groundwater.
Inter alia, that application introduced the basic process in which an electropotential gradient is applied, via two or more electrodes, to a bulk source containing target ions. The electropotential drives the target ions into an ion permeable host receptor matrix located between the electrodes. Upon reaching the host receptor matrix, the ions are immobilized and/or confined within the matrix, thereby permitting removal of the ions along with the host receptor matrix.
Subsequent research by the present inventors identified additional methods which are especially useful for mobilizing ions such as radionuclides which tightly adsorb to soil clays. (As described below, soil clays can present a particularly tough impediment for decontamination.) Those methods are described in copending U.S. application Ser. No. 07/683,973, filed Apr. 8, 1991. That application describes methods which use wave energy (e.g., microwaves, radio waves, sonic and ultrasonic waves, etc.) to effect or enhance the dissociation of ionic species from a soil matrix.
It is against this background that the inventions described herein are brought, in an effort to improve upon, and refine, the processes and apparatuses described in those earlier applications.
The '1392 application provides an extensive history of prior attempts at soil and groundwater decontamination, which will not be repeated here. For convenience, however, a brief review of such prior attempts, as well as some of the problems which spurred the developments described herein, are discussed in the following sections.
A. SOIL DECONTAMINATION METHODS
Prior attempts to decontaminate soil have variously included: (1) excavating the contaminated soil and processing it to remove the target ions; (2) providing an impervious covering over the ion contaminated regions, which essentially immobilizes the ion by preventing the intrusion of the groundwater necessary for ion mobility; (3) vitrifying the entire soil strata in-situ by adding a "frit" to the soil and establishing an intense electrical field which turns the soil into a glass-like mass which prevents leaching of the ion; (4) injecting a polymerizable monomer or ion exchange gel into the soil; (5) washing the soil with surfactant chemicals and/or pH adjusters to remove soil contaminants, and returning the cleansed soil; (6) excavating contaminated soil for burial at a remote site; (7) burying the contamination by deep plowing using special machinery; (8) changing the land usage; and (10) thermally processing the soil, e.g., through calcination or incineration.
In Chemistry and Industry, 18 Sep. 1989, pp. 585-590, Lageman et al. describe another variation of the in-situ treatment which involves an electrokinetic process. This method utilizes electrodes imbedded in the soil. The electrodes are accompanied by a chemical solution circulation system which circulates the chemical solution needed to minimize the electrode effects. The solution provides a mechanism for transporting the extracted contaminants to a central processing station. The concern for migration of chemical solution into the soil or groundwater is not adequately addressed by this process.
The foregoing methods, however, are regarded as only marginally effective in achieving their goal, namely the return of contaminated land to its original use.
B. GROUNDWATER DECONTAMINATION
Decontaminating groundwater can be extremely difficult. In some cases of contaminated underground aquifers, remediation is so impractical that the only economical solution involves identifying and isolating the source of contamination, and then delaying human contact until natural diffusion of water through the aquifer can provide for dilution of the contaminant.
In other instances, groundwater is treated with conventional ion exchange media, and/or charcoal absorber beds. Yet another option involves in-situ treatment in which chemicals are introduced which react with the ionic contaminants to form insoluble precipitates.
C. DECONTAMINATING RADIOACTIVE SOILS AND CLAYS
In the past, cesium contaminated soils have posed a major obstacle to soil cleanup programs. The difficulty relates to the low mobility of heavy metals such as cesium and plutonium in soil. It has been shown that clays provide the repository for deposited cesium. For example, Gale et al. in 1964 showed that 70% of the total cesium was within the 13% clay fraction of a sandy loam. Lomenick and Tamira in 1965 made measurements of lake sediments and concluded that 84% of the cesium was associated with the 35% clay fraction. Also, it has been found that interleaved mica, which is a constituent of this sediment, was the receptor for the cesium.
The significance of the above conclusion becomes apparent when considering that the cage structure of muscovite mica, which is a component of soil clays, is very similar to that of the zeolite, chabazite. Soil clays are present in a percentage ranging from 10-25 percent in virtually every fertile soil in the world.
Heretofore, removing such heavy radioactive species from the soils has been so difficult as to be considered impracticable. The reasons for this difficulty lie in the atomic characteristics of these heavy metals, especially in their ionic state ("M+"). Generally, the larger the M+ ion, the more numerous are its insoluble salts. One of the important properties of heavy metal ions is their tendency to become bound in a zeolite in insoluble form.
This phenomenon involves a cage-like ion trap found in zeolites, which is responsible for the zeolites often being referred to as an ion sponge or ion exchanger. An example is the cesium ion (Cs+) which is routinely encountered in radioactive form when dealing with fission-type nuclear power or nuclear weapons. The crystal radius of this cesium ion is 3.4 angstroms and the hydrated radius is 6.6 angstroms. The hydrated radius being the radius the cluster that consists of the ion and the water molecules which surround it. The zeolite mineral of the chabazite class has receptor sites whose pore size ranges from 3-7 angstroms which is such that the cesium (without its hydration layer) will fit nearly perfectly into this "host-cage structure". By comparison, a smaller ion (sodium for example) would tend to be more weakly bound in the cage, and upon the arrival of a cesium ion, would be easily displaced.
D. DECONTAMINATION USING ION EXCHANGE
Many of the currently employed methods for decontamination of water and soils involve some form of diffusion-controlled ion exchange.
For example, one of the world's largest plants for treatment of effluent from a spent nuclear fuel plant is the British Nuclear Fuels, Inc., SIXEP Plant. The SIXEP process is based on ion exchange using an inorganic ion exchanger. In this process, the positively charged cesium and strontium ions are taken up into the crystal lattice of clinoptilolite (a class of porous crystalline aluminosilicates of the zeolite family), in preference to the sodium ions which are naturally present. In this process, water containing the radioactive ions is caused to pass into close proximity to the clinoptilolite, whereupon diffusion takes place to cause migration of the radioactive ions into the clinoptilolite lattice, displacing the sodium ions.
In more recent technology, novel electrochemical cells incorporating ion exchange membranes have been used to rid water of metal ion contaminants in an economical manner. However, such cells generally have not been favored because the waste form (or concentrated effluent) is liquid, which is less desirable than solid waste.
The main disadvantage of the existing electrodialysis technology is that its use in an in-situ remediation or cleanup of either soil or groundwater is extremely limited, partly because of the presence of ionic colloids which will "blind" or plug the membrane, and partly due to the limitation in transport of a "complexed" ion species. Also, these cells can involve relatively complex operation, substantial capital investment in operating hardware, and a liquid waste form which in some cases can comprise a very large volume of waste. Most importantly, none of these processes are able to remove soluble ions from a bulk source.
E. TECHNOLOGY OF THE '1392 APPLICATION
The basic technology of the '1392 application represented a significant advance over prior decontamination techniques. Nevertheless, as is the case with most new technology (and most inventors), the desire for further refinement and improvement of their earlier technology accompanied the present inventors' subsequent research. Inter alia, two areas of interest prompted the research leading to the inventions discussed herein.
First, the present inventors realized that the '1392 application processes and systems were not achieving maximum efficiency. They discovered that inefficiency in the system could be due to the presence of unwanted hydrogen (H.sup.+) and/or hydroxyl (OH.sup.-) ions in the bulk source. Those ions, which are generated in large quantities at the anode and cathode, are relatively small and thus much more mobile than the larger ions which are the target of decontamination. Once formed, they can migrate rapidly, via the current generated by the electrodes, to the opposite electrode. Further, the generation of hydroxide ions at the cathode raises the pH in the vicinity of the cathode which causes precipitation of the contaminant cations as metal hydroxides. Thus, it was realized that much of the current being generated by the electrodes was being spent on moving the hydrogen and hydroxyl ions, and not the target ions. Further still, an efficient means of isolating the contaminants at the electrodes would be needed.
Secondly, the present inventors realized that treating large areas of contaminated land, such as the land surrounding Chernobyl, required a system having several properties. First, the system should be designed for use where most of the contamination is relatively near the surface. Second, the system should be designed to be able to cover large surface areas, and receive and confine large volumes of contaminants. Third, the system should be designed to be reusable and/or recyclable. Lastly, the system should be relatively mobile such that it readily can be moved to a new area when decontamination of the current area is completed.